| Artificial heart polyurethane diaphragms are subjected to cyclic loading which makes devices scheduled for long-term use prone to fatigue failure. Stress concentrations developed during the deformation of the diaphragm are the sources of microcracks that lead to the untimely fatigue failure. A solid model of a typical diaphragm shape was analyzed for stress and strain using Finite Element Analysis (FEA) software, ABAQUS/Standard. The Mooney-Rivlin, hyperelastic, strain energy model for strains <50%, is evaluated for use in the FEA analysis. The material constants for the strain energy model can be determined from standard homogenous material tests, uniaxial, biaxial and planar materials tests. For this study, the material constants were determined from uniaxial stress-strain data and biaxial stress-strain data for biomedical grade polyurethane. Uniaxial data was provided from a local manufacturer of the polyurethane material. For biaxial data, an inflation apparatus was created to determine the biaxial stress-strain curve for the same material. Planar test data was excluded from the analysis. The results from both data sets were evaluated for use with the Mooney-Rivlin curve and are compared against published data for Biomer. A step-by-step analysis of ABAQUS/Standard was undertaken to assess boundary conditions, analysis methods and element types for 3D analysis of the polyurethane diaphragm. A structured hexagonal mesh using second order, hybrid, reduced integration elements, C3D20RH and a built-in boundary condition is recommended for use in the diaphragm inversion and deformation analysis. The model is expected to form the basis of future efforts investigating more complex shapes. This study found strains over 50% and stresses over 5MPa appear near the periphery of the model when the diaphragm is inverted and pressurized to 30KPa (255 mmHg). |
